Clusters in carbon martensite: Thermodynamics and kinetics

An original method of evaluation of the cluster population in carbon marten site has been developed. Using this method, it is shown that Kurdjumov's mo del of carbon redistribution within the different octahedral site sublattic es can quantitatively account for both observed normal and abnormal tetrago nality in carbon martensite. It is also shown that the existence of the int ernal strains in martensite constitutes a necessary and sufficient conditio n for the energetic preference of tetrahedral over the cubic lattice. The p resence of the residual tetragonal distortion in the quasi-cubic phase of k appa-martensite is associated with the presence of the mixed clusters forme d of the atoms belonging to O-c sublattice as well as to remaining ones. By using a computer simulation of the dynamical behavior of carbon martensite approaching the thermodynamical equilibrium, it was found that the ultimat e state of this system is strongly beyond the thermal equilibrium. Even aft er long-term aging, the free energy, is far beyond the minimum value allowe d for this system. The reason for such a behavior and the possible aging pr ocesses proceeding in this system are discussed at the molecular level. All of the ordering parameters are affected by the aging process. The evolutio n proceeds in the distinctly different time intervals for different paramet ers. At first, the long-range ordering parameter that determines the tetrag onality of martensite evolves and reaches the stable value. In the next sta ge, the formation and then disintegration of two-particle clusters occurs. Disintegration of two-particle clusters coincides with the stage when three -particle cluster formation occurs at a high rate. Three-particle clusters also disintegrate when some time elapses. The same pattern repeats regardin g four, five-, six-, seven-, and eight-particle clusters. To simplify the c alculations, the nine-particle clusters are assumed to be the largest possi ble and are identified with an existence of superstructure. The formation o f 100 pct of nine-particle clusters with no contribution of free atoms in a n alloy ceases all aging processes. The evolution of these processes is ill ustrated graphically in the time range from 16 seconds to 1500 years, as es timated on the basis of experimental data.